1. Field
This patent specification relates to an optical pickup apparatus which records/reproduces data on different kinds of optical recording media by using selectively laser light of different wavelengths.
2. Discussion of Related Art
In a pickup apparatus, a laser light from a laser diode is incident onto a signal recording surface of an optical recording medium by way of, for example, a hologram device, and light reflected from the signal recording surface is detected by a photodetection device by way of the hologram device, thereby recording/reproducing on an optical recording medium such as DVD or CD. The laser light is controlled to maintain an optimum level by monitoring an output level of the laser light in order to record/reproduce stably.
Therefore, conventionally, as illustrated in FIG. 9, a photodetection device for monitor 8 is arranged on an optical path in an optical pickup apparatus, and an output level of a laser light emitted from a laser diode 1 is detected by a photodetection device for monitor 8 and the control is performed on the basis of a detected value so that the laser light can be maintained at an optimum level. Numeral 3 is a hologram device and numeral 4 is a collimator lens.
However, in this method, a photodetection device for monitor 8 in addition to a photodetection device for the main signal needs to be arranged. The photodetection device for monitor 8 needs to fit in a limited space, thus complicating the design and, moreover, a detection amount of the photodetection device for monitor 8 changes greatly by inaccuracies in its positioning.
In order to deal with this problem, as illustrated in FIG. 10, a method for detecting the level of the laser light from the laser diode 1 is adopted such that a reflection hologram 9 is formed on the hologram device 3, and the laser light from the laser diode 1 is reflected and diffracted by the reflection hologram 9 and is detected by a photodetection device 7. In this method, the photodetection device for the main signal and the photodetection device for monitoring can be formed on one plate, and the number of devices is reduced and further the photodetection device is simply adjusted.
In this case, when laser light of different wavelengths (635 nm or 650 nm, and 780 nm) from the laser diode 1 and a laser diode 2 are used, the laser light of 635 nm or 650 nm from the laser diode 1 and the laser light of 780 nm from the laser diode 2 pass the hologram device 3 as illustrated in FIG. 11A, and a reflecting type-diffraction element 10a which reflects the laser light of 635 nm or 650 nm and a reflecting type-diffraction element 10b which reflects the laser light of 780 nm are formed.
(In this patent specification, the term “laser light” refers to a beam (or beams) of light emitted from a laser (of from lasers), reflected by an optical recording medium, and/or acted on by devices such as lenses, diffraction grantings, etc.
Each of the reflection light from the reflecting type-diffraction element 10a and the reflection light from the reflecting type-diffraction element 10b is guided to and detected on a photodetection element for monitor 7m formed in the photodetection device 7, and the output levels of the laser light from the laser diodes 1 and 2 can be monitored respectively.
When the output levels of laser light of the different wavelengths described above are monitored, the laser diodes 1 and 2 are spaced apart each other and the laser light emitted from a collimator lens 4 proceeds in a slant direction, and is incident to an objective lens slantingly, and thus aberration occurs in a spot formed on the optical recording medium
Therefore, the laser diodes 1 and 2 can be very close to each other, and still the emitted patterns from the laser diodes 1 and 2 on the surface of the hologram device 3 would be as illustrated in FIG. 11A. Thus, for example, when the laser light 1 is illuminated, the laser light of 635 nm or 650 nm is emitted, however, the laser light is incident not only onto the reflecting type-diffraction element 10a but also the reflecting type-diffraction element 10b as illustrated in FIG. 11B. Numeral 25a is a diffraction light from the diffraction element 10a and numeral 25b is a diffraction light from the diffraction element 10b. 
In this case, as illustrated in FIG. 11C, the reflection light from the reflecting type-diffraction device 10a forms a small spot 26a and focuses, and the output level of the laser light from the laser diode 1 can be detected. On the other hand, the reflection light from the reflecting type-diffraction element 10b becomes a large spot 26b before focusing on the photodetection device 7, and is incident onto a different photodetection element 27 and thereby a flare light occurs. As a result, the detection accuracy of the output level of the laser light from the laser diode 1 can decrease.